1. Field of the Invention
This invention relates to a handpiece for use in medical treatment, especially in dental treatment, and more particularly to improvement in a dental handpiece having a brushless motor incorporated thereinto for driving a cutting tool at the end of the handpiece and having a cooling means for the teeth and the cutting tool.
2. Prior Art
A motor-driven type dental handpiece includes a cutting tool for the teeth at the end of the grip of the handpiece and is designed to drive the tool by the use of a motor incorporated into the tool and to cool both the teeth and the tool by feeding a coolant such as water or air during the cutting operation.
In the structure described above, when a coolant passageway is formed exposedly on the outside of a handpiece body, the handpiece becomes uneven in diameter throughout its length and large in size as a whole, making it uncomfortable to grip the handpiece and hence reducing operability. This is not desirable for treatment activities.
Accordingly, it is proposed to provide a coolant passageway inside the handpiece body, but this inevitably results in passing the coolant passageway through a micromotor portion, which fact produces no small effect on the magnetic path of the motor, leading to trouble such as a reduction in motor torque, production of torque ripple, etc.
As shown in a typical front view in FIG. 1, the brushless motor is generally constructed in such a manner that a necessary clearance D is formed between a coil B fixed inside a cylindrical yoke C and a motor magnet A so as to provide a magnetic path indicated by arrows in the drawing. In the motor, heretofore a coolant passageway E has been formed linearly on the outer periphery of a cylindrical yoke C and in parallel to the axis of the yoke C as shown in FIG. 2. Accordingly, the sectional area of the yoke C is largely reduced at the portion C.sub.1 --C.sub.1 shown in an exploded view of FIG. 3 to provide a sectional area view in FIG. 4. The sectional area S of the yoke at the portion C.sub.1 --C.sub.1 is expressed by the formula below: EQU S=(a-b.sub.1).times.l
In this formula, a represent the thickness of the yoke; b.sub.1 the groove thickness of coolant passageway; and l the length of the yoke. For example, if a=1.9 m/m, b.sub.1 =0.8 m/m and l=49 m/m, the sectional area is reduced by about 12%. Therefore, the yoke C produces a magnetic saturation phenomenon in the portion wherein the sectional area is reduced, so that the normal magnetic passageway as shown in FIG. 1 is not formed and torque is reduced. The reduction in torque results inevitably in an increase in the size of the micromotor to compensate for the amount of reduction in torque, and in turn causes an increase in the diameter of the handpiece.
When observation is made of the magnetic flux distribution during rotation of the rotor magnet A in the direction of I.fwdarw.II.fwdarw.III.fwdarw.IV in FIG. 5 in the micromotor having the coolant passageway formed therein, the magnetic flux distribution on the right and left with the magnet A therebetween is rendered unsymmetric at points I and III by being hindered by a coolant passageway E and is rendered symmetric at points II and Iv as is apparent from the drawing. Torque ripple is produced by such fluctuation in the magnetic flux distribution. The torque ripple thus produced deteriorates motor characteristic, constituting a direct obstacle to dental treatment.